1. Field of the Invention
The present invention relates to an image forming apparatus for forming a color image.
2. Description of the Related Art
In a known image forming method using electrophotographic process, a color image is formed in the following manner.
An electrostatic latent image is first formed on a latent image-holding member, and the electrostatic latent image is developed to a developed toner image with a developing device. The developed toner image is then transferred onto an intermediate transfer medium. The electrostatic latent images for different toner colors are consecutively formed and developed, and the developed images are transferred superimposed on the intermediate transfer medium, to form a full color toner image. The full color toner image transferred on the intermediate transfer medium is then transferred on a recording medium such as paper, and then fixed thereon.
The image forming apparatus of forming a color image by the method had a problem of the contamination of color toners in developing devices due to incorporation of one color toner to the other color toners and the resulting color mixing which is caused by scattering of the one color toner from both terminals of the toner carrier in the developing device or from the toner-supplying port during replenishment of the toner into the developing device.
In particular, in a rotary development system of placing a rotary development unit containing multiple developing devices at a position facing a latent image-holding member, moving one of the multiple developing devices selectively to the position facing the development position of the latent image-holding member by rotation of the rotary development unit, and developing the electrostatic latent image with a toner by rotating the toner carrier in the developing device and applying a development bias, such color mixing occurs more frequently, because the position of the developing devices vary.
For example, when a black image is developed in such a rotary development system, a black developing device moves to the position facing the latent image-holding member by rotation of the rotary development unit. A black toner carrier is rotated more frequently because the black image is printed more frequently than images in other colors. As a result, the black toner is scattered more frequently from both terminals of the toner carrier. In addition, the black toner is replenished frequently, making easier scattering of the toner also from the toner-supplying port. When a color image in cyan, magenta, and yellow is developed, for example, the developing device in each color moves to the position facing the latent image-holding member by further rotation of the rotary development unit, the black toner scattered from the black developing device deposits easily on the developing carrier in the developing device in other color by rotation of the rotary development unit. When the black toner deposits on the developing carrier of other color toner, the black toner causes color mixing in the color image area, leading to image staining.
A method of improving the apparatus was proposed for prevention of the toner leakage. For example, Patent Document 1 (Japanese Unexamined Patent Publication No. 2001-134045) and Patent Document 2 (Japanese Unexamined Patent Publication No. 2005-316204) disclose image forming apparatuses in which the sealing efficiency of the rotation part for the toner-supplying unit placed outside a developing device is improved and the drive load of the rotary development unit during rotation is lowered. However, improvement in seal efficiency leads to increase of the load to the rotary development unit, making it difficult to prevent toner leakage completely.
On the other hand, for example, Patent Document 3 (Japanese Unexamined Patent Publication No. 2003-107829) discloses a method of using polymerized toners as color toners and a grinded toner as black toner. However, the method is effective in preventing staining of the fixing part, but not so effective in preventing the image staining caused by contamination of the toner in developing devices. Alternatively, for example, Patent Document 4 (Japanese Patent 3347646) and Patent Document 5 (Japanese Unexamined Patent Publication No. 9-15925) disclose methods of using a magnetic component as black toner and nonmagnetic toners as color toners, but none of the methods could not solve the problem of image staining caused by color mixing in the color-developing device.